The Quicksort Sorting Algorithm: Pick A Pivot, Partition, & Recurse

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Question: Implement QuickSort. Explain how it works. Explain the best and worst cases.
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[@BackToBackSWE]

Thanks. I haven't burnt out. I just started work in San Francisco and I am building a pretty cool and fancy site for this channel which I hope to launch mid-end of July. Once it is out I'll be back on the throttle to post more technical videos as I support the site and begin working on the next software.

[@SihaLoire]

FINALLY someone who explains what a pivot is. Pivot concept is key to understand how quicksort works and also to understand "the kth largest element in an array" problem.

[@BackToBackSWE]

yes

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"We do not place an item, into the section, that is less than the pivot." - just needed that much. Thank You, Brother! #keepUptheGoodWork.

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great to hear.

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When did I ask lol

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I have never commented on a KZbin video before but thank you so much for this! Your code for Quicksort actually makes sense, which is something I have not found to be the case for other sources, including Cracking the Coding Interview. I have subscribed :)

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ye

[@rogerrodulfo]

probably the most easy to understand video on quicksort. Thank you

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Perfect! I really love that, you explain it as good as it's posssible. Nice.

[@BackToBackSWE]

Table of Contents: Talking About Stuff 0:00 - 1:05 Introducing Quicksort 1:05 - 2:04 The Split Subroutine 2:04 - 2:43 The Partition Subroutine 2:43 - 3:24 Partitioning: Choosing A Bad Pivot 3:24 - 10:34 Partitioning: Choosing A Good Pivot 10:34 - 15:11 Analysis: Choosing Bad Pivots 15:11 - 19:06 Analysis: Choosing Good Pivots 19:06 - 22:07 Analysis: Good Pivots - Work Done At Leaf Level 22:07 - 23:37 Flashback To Merge Sort 23:37 - 24:01 Why Quicksort Is O(n*log(n)) 24:01 - 25:02 Wrap Up 25:02 - 26:12 Errors: 3:25 - 15:11 -> I messed up how I advanced 'i' in both examples. I should have started 'i' at left - 1 everytime...my bad. What should happen is that we should start 'i' at 'left - 1' and anytime we swap into 'i' we always advance 'i' FIRST before the swap. These errors don't change the fundamental job of 'i', which is to keep the tail of the section of items less than the pivot. Make sure you take that away from this. 4:02 -> Forgot to edit that text out haha 24:23 -> This is only the exact bound on the best case. Not the average case as well. The code for quicksort is in the description. Fully commented for teaching purposes.

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thanks

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I haven't understood quicksort for 2 years, thanks a bunch man

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Finally understand quick sort! Thanks so much

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Thanks for this, I was actually wondering couple days ago why there isn't a sorting video for quick sort.

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This is the only vedio I found which tells exactly what exactly is PIVOT.. THANKS BRO

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Hey, my friend. I'm an SDE with a couple of years of industry and academic background. I really appreciate your effort to explain this algorithm. Regardless of the commercial purpose behind the video, your explanation is mostly accurate and easy to understand ( Though I have to say, it's better for you to put the average case before the worst-case example in this video.) Carry on, looking forward to new content from you! btw: I have the confidence to say many, yes many, explanations of quicksort from some others are WRONG. Well done, sir!

[@lomoyang3034]

one more suggestion, I will be better for you to explain why quick sort is "Quick". I know the answer..but..I'll leave the work to you...

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Really thanks for the detailed explanation, this video solved my questions watching Wiki of Quicksort

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thanks

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best explanation on quick sort !!

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I really felt the "move on" at 7:37. Btw, really helpful video!

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In middle school I think...near corn fields 🌽🌽

[@mattstyles4283]

Amazing videos, the only teacher who actually explains algorithms clearly! One question- at 11:19 you said that "it does matter where i goes... I messed up... but you get the idea". Where should i go, at -1 or 0?

[@mattstyles4283]

I saw in your attached code that i starts at -1, so that when j finds a number, i will be incremented to point to the next available spot in the 'less-than' portion. Then the swap occurs, and j is advanced forward. However, I've also been messing around with my own implementation and it seems that you can start i at 0, as long as you perform the swap first and THEN increment i. When it comes to reinserting the pivot, it needs to be at i (rather than at i+1 when i starts at -1).

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